1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device and a method for producing the same. In particular, the present invention relates to an improvement in a discharge auxiliary electrode provided for the purpose of accelerating electrostatic discharge in an ESD protection device.
2. Description of the Related Art
An overvoltage protection element that is interesting to the present invention is described in, for example, Japanese Unexamined Patent Application Publication No. 2008-85284 (Patent Document 1).
Patent Document 1 describes an overvoltage protection element material which is to become a discharge auxiliary electrode provided for the purpose of accelerating discharge. The material contains a non-conductor powder (for example, silicon carbide: particle size 1 to 50 μm) and a metal conductor powder (for example, copper: particle size 0.01 to 5 μm), and an adhesive (for example, glass powder).
In addition, Patent Document 1 describes a method for producing an overvoltage protection element, the method including a step of preparing a material paste by uniformly mixing a non-conductor powder, a metal conductor powder, and an adhesive at a particular ratio, a step of printing the material paste on a substrate, and a step of conducting a firing process (temperature: 300° C. to 1,200° C.) on the substrate.
However, the technology described in Patent Document 1 has the following problems to be solved.
First, since the surface of the metal conductor powder is exposed, exposed metal conductor particles are bound to each other during discharge, which may decrease insulation reliability. Furthermore, silicon carbide, which is used as a non-conductor powder, is a semiconductor having a relatively low insulation resistance, and thus it is difficult to improve insulation reliability.
For example, International Publication No. 2009/098944 pamphlet (Patent Document 2) describes a technology that can solve the problems described above.
Patent Document 2 describes the use of a discharge auxiliary electrode in which a conductive material (such as a Cu powder) coated with an inorganic material (such as Al2O3) is dispersed. According to the technology described in Patent Document 2, insulation reliability can be enhanced because the exposure of the conductive material is suppressed as compared with the technology described in Patent Document 1. Furthermore, even when the content of the conductive material is increased, short-circuit between particles of the conductive material does not easily occur. Thus, discharge occurs more easily by increasing the content of the conductive material. As a result, the peak voltage can be decreased.
However, the technology described in Patent Document 1 also has the following problems to be solved.
The “conductive material coated with an inorganic material” in the technology described in Patent Document 2 is merely a material obtained by coating the surface of a conductive material with fine particles composed of an inorganic material, as described in paragraphs [0034] and [0094] and FIG. 4 in Patent Document 2. Accordingly, it is relatively difficult to completely cover the surface of the conductive material with the inorganic material. Furthermore, even if the surface of a conductive material is completely covered with an inorganic material before the stage of firing, as shown in FIG. 12, when the conductive material 1 is thermally expanded during firing, the conductive material 1 is not completely covered with the inorganic material 2 and the conductive material 1 may be exposed after firing. Therefore, regarding insulation reliability, a further improvement has been desired.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-85284    Patent Document 2: International Publication No. 2009/098944 pamphlet